1. Field of the Invention
The invention relates in general to waveguide circulators for the non-reciprocal transmission of microwave energy; and more particularly to a novel approach for reducing the size, insertion loss, and cost of a waveguide switching circulator.
2. Description of the Related Art
Ferrite circulators have a wide variety of uses in commercial and military, space and terrestrial, and low and high power applications. A waveguide circulator may be implemented in a variety of applications, including but not limited to low noise amplifier (LNA) redundancy switches, T/R modules, isolators for high power sources, and switch matrices. One important application for such waveguide circulators is in space, especially in satellites where extreme reliability is essential and where size and weight are very important. Ferrite circulators are desirable for these applications due to their high reliability, as there are no moving parts required. This is a significant advantage over mechanical switching devices. In most of the applications for waveguide switching and non-switching circulators, small size, low mass, and low insertion loss are significant qualities.
A commonly used type of waveguide circulator has three waveguide arms arranged at 120° and meeting in a common junction. This common junction is loaded with a non-reciprocal material such as ferrite. When a magnetizing field is created in this ferrite element, a gyromagnetic effect is created that can be used for switching the microwave signal from one waveguide arm to another. By reversing the direction of the magnetizing field, the direction of switching between the waveguide arms is reversed. Thus, a switching circulator is functionally equivalent to a fixed-bias circulator but has a selectable direction of circulation. Radio frequency (RF) energy can be routed with low insertion loss from one waveguide arm to either of the two output arms. If one of the waveguide arms is terminated in a matched load, then the circulator acts as an isolator, with high loss in one direction of propagation and low loss in the other direction.
Generally, these three-port waveguide switching circulators are impedance matched to an air-filled waveguide interface. For the purposes of this description, the terms “air-filled,” “empty,” “vacuum-filled,” or “unloaded” may be used interchangeably to describe a waveguide structure. Conventional three-port waveguide switching circulators typically have one or more stages of quarter-wave dielectric transformer structures for purposes of impedance matching the ferrite element to the waveguide interface. The dielectric transformers are typically used to match the lower impedance of the ferrite element to the higher impedance of the air-filled waveguide so as to produce low loss. There are several disadvantages to using transformers in such a manner. When dielectric transformers are used, RF losses can be introduced in various ways, such as the following: losses in the dielectric material itself, increased losses in the waveguide surfaces due to the high concentration of RF currents on the metal waveguide surfaces disposed directly above and below the dielectric transformer element, and losses in the adhesives typically used to bond the transformers to the conductive housing. Importantly, the use of dielectric transformers also takes up additional space in the waveguide structure, thereby increasing the minimum size and weight of the circulator.
Previous patents (U.S. Pat. No. 4,697,158; U.S. Pat. No. 3,277,399; U.S. Pat. No. 4,058,780, Pub. No. WO 02/067361 A1) have described approaches for achieving broad bandwidth through the addition of impedance matching elements. Broadband circulators have high isolation and return loss and low insertion loss over a wide frequency band, which is desirable so that the circulator is not the limiting component in the frequency bandwidth of a system. Broad bandwidth also allows a single design to be reused in different applications, thereby providing a cost savings. These prior art approaches for achieving broad bandwidth generally involve the addition of quarter-wave dielectric transformers or steps in the height or width of the waveguide structure to thus achieve impedance matching of the ferrite element to the waveguide port. For example, U.S. Pat. No. 4,697,158 discloses achieving impedance matching by providing a step or transition in the waveguide pathway. This technique eliminates the standard dielectric transformers, but is very sensitive to dimensional variations, resulting in a design that is difficult and expensive to manufacture reliably. This design also relies on the presence of a significant gap or spacing between adjacent ferrite elements, increasing the size and weight of the structure. These methods all require impedance matching elements in addition to the ferrite element in order to achieve acceptable performance. Other patents, such as U.S. Pat. No. 5,724,010, discuss changing the shape of the ferrite resonant structure to achieve broadband performance. However, these ferrite structures are restricted to fixed-bias applications with a single direction of circulation. Lastly, my co-pending patent application, Publication US2003/0107447 titled MUILTI-JUNCTION WAVEGUIDE CIRCULATOR WITHOUT INTERNAL TRANSITIONS (the '7447 application publication), incorporated herein by reference, describes implementations wherein the impedance matching elements have been eliminated between adjacent ferrite elements by careful selection of the width of the waveguide pathway in the region of the ferrite elements and the use of a de minimis air gap (less than a fraction of a wavelength) between the adjacent ferrite elements. That application, however, describes use of conventional quarter wavelength dielectric transformers for those protruding parts of the circulator (which in a Y-shaped element are referred to as legs) not directly adjacent to another protruding part of an adjacent circulator. The '7447 application claims many of the same benefits of improvements in size, loss, and complexity of this new invention, but the prior approach utilizes the symmetry in the adjacent ferrite elements for impedance matching. This technique is only applicable between two adjacent ferrite elements, and the embodiments disclosed therein still utilize the traditional quarter-wave dielectric transformers for impedance matching in the circulator legs that do not face or abut an adjoining ferrite junction. Accordingly, it can be seen that a need yet remains in the art for a circulator with reduced size, weight, and cost and which can avoid the need for separate elements for impedance matching. Therefore, the present invention is directed to providing such a broadband switching circulator wherein performance can be achieved between a ferrite junction and a waveguide port interface without the addition of impedance matching elements.